This invention relates to magnetorheological fluids.
Magnetorheological (MR) fluids are substances that exhibit an ability to change their flow characteristics by several orders of magnitude and in times on the order of milliseconds under the influence of an applied magnetic field. These induced rhleological changes are completely reversible. The utility of these materials is that suitably configured electromechanical actuators which use magnetorheological fluids can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, brakes for controllable suspension systems, vibration dampers in controllable power train and engine mounts and in numerous electronically controlled force/torque transfer (clutch) devices.
MR fluids are noncolloidal suspensions of finely divided (typically one to 100 micron diameter) low coercivity, magnetizable solids such as iron, nickel, cobalt, and their magnetic alloys dispersed in a base carrier liquid such as a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluids have an acceptably low viscosity in the absence of a magnetic field but display large increases in their dynamic yield stress when they are subjected to a magnetic field of, e.g., about one Tesla. At the present state of development, MR fluids appear to offer significant advantages over other types of controllable fluids, such as ER fluids, particularly for automotive applications, because the MR fluids are relatively insensitive to common contaminants found in such environments, and they display large differences in rheological properties in the presence of a modest applied field.
A typical MR fluid in the absence of a magnetic field has a readily measurable viscosity that is a function of its vehicle and particle composition, particle size, the particle loading, temperature and the like. However, in the presence of an applied magnetic field, the suspended particles appear to align or cluster and the fluid drastically thickens or gels. Its effective viscosity then is very high and a larger force, termed a yield stress, is required to promote flow in the fluid.
Because MR fluids contain noncolloidal solid particles which are at least five times more dense than the liquid phase in which they are suspended, suitable dispersions of the particles in the liquid phase must be prepared so that the particles do not settle appreciably upon standing nor do they irreversibly coagulate to form aggregates. Without some means of stabilizing or suspending the solid, sedimentation and/or flow induced separation of the solid phase from the liquid phase will occur. Such separation will have a drastic and detrimental effect on the ability of the MR fluid to provide optimal and repeatable performance.
The magnetizable particles are kept in suspension by dispersing a thixotropic agent in the liquid vehicle. There are basically two approaches to the stabilization of MR fluids: the use of polymeric thickeners, such as high molecular weight hydrocarbons, polyureas, etc., or the use of a finely divided solid, such as fumed silica or colloidal clay. Essentially, both approaches aim to prevent separation of the liquid and solid phases by forming a thixotropic network which xe2x80x9ctrapsxe2x80x9d or suspends the heavier solid in the lighter liquid. Of these two methods, the use of polymeric thickeners in MR fluids can be problematical, since it is difficult to achieve sufficient stability against settling without using an amount of thickener which will impart a grease-like consistency to the composition. Although sedimentation or settling is minimized, the MR fluid is no longer free flowing, and in fact, may exhibit an unacceptably high viscosity.
An alternative to polymeric thickeners is fumed silica. It has been demonstrated in the prior art that fumed silica can be used as a stabilizer in MR fluid compositions, provided attention is given to the selection of fumed silica grades that are compatible with the chemistry of the liquid phase. This selection is complicated by the fact that the liquid phase is often a combination of miscible, but chemically different materials. If adequate shear mixing is achieved in processing, a lightly gelled system can be formulated using fumed silica. Although characterized by a xe2x80x9cyield stressxe2x80x9d (defined as the applied force/area required to initiate flow) sufficient to prevent settling, it has been shown that such a system will still flow with a moderate to low viscosity. However, one perceived disadvantage in using fumed silica is that this material, even in amounts less than two or three percent/volume, can cause the MR fluid to be abrasive towards polymeric seals as well as metallic wear surfaces in the device. This is particularly detrimental in vehicle damper applications, where a considerable amount of expense and effort has been devoted to providing wear-resistant coatings, for example, to protect the damper from failure due to excessive wear. Also, there is growing evidence that fumed silica is a key factor contributing to xe2x80x9cin-use thickeningxe2x80x9d, or paste formation, of MR fluids in suspension dampers subjected to accelerated durability testing.
An alternative approach to polymeric thickeners and fumed silica, both of which have potential drawbacks in formulating MR fluids, is to use colloidal clay. Using a surface-treated, colloidal organoclay as a stabilizer for MR fluids was first demonstrated and patented in U.S. Pat. No. 6,203,717 by Lord Corporation, and forms part of the package for the MR fluid (B5.2F) which, for example, has been approved for vehicle shock absorber production. In contrast to polymeric thickeners, and similar to fumed silica, an MR fluid with the organoclay will form a light gel at low volume concentrations, with a yield stress sufficient to prevent or significantly retard settling, but with an ability to flow with low to moderate viscosity. Moreover, the clay is inherently less abrasive than fumed silica, suggesting the possibility to reduce expensive surface treatments used to retard or prevent abrasion.
Although organoclay stabilizer systems for other applications (lubricating greases, cosmetics, etc.) are known, and are even being utilized in vehicle applications, there are still significant performance issues impacted by the organoclay which need to be addressed. Essentially, as is the case in any technology which relies heavily on surface chemistry to achieve a desired effect, the particular surface treatment of the organoclay must be chosen carefully to insure compatibility with the liquid phase, as well as to achieve a balance between interactions which contribute to yield stress, and those which contribute to viscosity. It would be highly desirable to achieve a desired level of yield stress independently of viscosity. The method disclosed in the Lord patent (U.S. Pat. No. 6,203,717) of using a single organoclay to achieve stability against settling in a hydrocarbon liquid vehicle, with easy redispersibility of any sediment that does occur, involves trade-offs. To achieve a reasonable yield stress for stabilizing the system, a clay is chosen from among those commercially available products which are compatible with the liquid phase, which in the Lord patent is a non-polar synthetic hydrocarbon. However, for damper fluids with stringent seal swell and volatility requirements, the liquid phase is advantageously a mixture of a non-polar synthetic hydrocarbon and a polar diester. Due to the character of the liquid phase, and the fact that commercially available organoclays are designed to be compatible with a given class of liquids of a given polarity, rather than mixtures, a very short list of compatible materials results. A final material is then chosen on the basis of screening with respect to settling and viscosity. Not surprisingly, the resulting compromise often leaves the system marginalized with respect to a low yield stress, and a moderate viscosity. In addition, the yield stress and/or viscosity is often sensitive to the addition of other required components, such as anti-wear additives and antioxidants, requiring adjustment of the clay level to compensate.
There is thus a need for an organoclay stabilizing system that is compatible with the liquid mixture used in many MR fluids so as to decouple the yield stress and viscosity, allowing the optimizing of each property more or less independently.
The present invention provides a magnetorheological fluid formulation comprising magnetizable particles dispersed in a liquid vehicle mixture comprising at least two liquid components of different surface functionality and an organoclay stabilization mixture. In accordance with the present invention, at least one organoclay is selected for each liquid vehicle component, each organoclay having a surface chemistry that renders it preferentially compatible with the surface functionality of one of the liquid components relative to its compatibility to the remaining liquid components whereby it is effective to stabilize, or gel, that component. By using the organoclay stabilization mixture of the present invention, the yield stress and viscosity of the MR fluid may be independently controlled, and the magnetizable particles remain suspended in the liquid vehicle. There is further provided a method of making an MR fluid in which liquid vehicle components are blended together, the organoclay mixture is added to the blend, and magnetizable particles are suspended therein, resulting in a stable MR fluid of suitable viscosity and yield stress.